The importance of reliable, operational definitions of mental disorders is well recognized; the advent of the research diagnostic criteria approach significantly advanced research studies and was subsequently adapted for both clinical and research purposes in DSM III. Controversy continues to surround issues of how diagnostic criteria should be developed and validated, particularly in relation to psychiatric disorders of childhood. In this project we propose the implementation of rigorous methodological procedures in the validation of diagnostic criteria for autism. Although autism has appeared to constitute probably the most robust example of a "disease entity" in child psychiatry considerable controversy has surrounded the definition of the syndrome, as exemplified in DSM III and III-R. Application of diagnostic criteria in DSM III and III-R result in significantly different patterns of diagnostic assignment. In this project examine basic psychometric properties of diagnostic criteria for the disorder using criteria from DSM III, III-R, and ICD-10 will be examined. Aspects of reliability (interrater, test-retest, and internal consistency) will be assessed. Psychometric properties of individual criteria will be evaluated, i.e., they will be treated as individual items in a psychological assessment instrument. Innovative methodologies in this area (signal detection theory, factor analysis of phicoefficients, item weighting, and Rasch scaling) will be used to develop a set of best diagnostic criteria. The applicability of these criteria will then be assessed in a replication sample. Patterns of agreement/disagreement between diagnostic schemes will be assessed. Factor and cluster analytic procedures will be used to evaluate commonly proposed subtyping schemes. This study will make use of the special clinical resource provided by the availability of a large cohort of autistic and nonautistic but developmentally disabled cases followed may have much wider applicability to other psychiatric disorders and may facilitate the development of DSM IV.